Ground-referenced single-ended signaling connected graphics processing unit multi-chip module

ABSTRACT

A system of interconnected chips comprising a multi-chip module (MCM) includes a first processor chip, a graphics processing cluster (GPC) chip, and an MCM package configured to include the first processor chip, the GPC chip, and an interconnect circuit. The first processor chip is configured to include a first ground-referenced single-ended signaling interface circuit. A first set of electrical traces fabricated within the MCM package and configured to couple the first single-ended signaling interface circuit to the interconnect circuit. The GPC chip is configured to include a second single-ended signaling interface circuit and to execute shader programs. A second set of electrical traces fabricated within the MCM package and configured to couple the second single-ended signaling interface circuit to the interconnect circuit. In one embodiment, each single-ended signaling interface advantageously implements ground-referenced single-ended signaling.

CLAIM OF PRIORITY

This application is a continuation-in-part of U.S. application Ser. No. 13/844,570, filed Mar. 15, 2013, the entire contents of which is incorporated herein by reference.

This invention was made with Government support under Agreement No. HR0011-10-9-0008 awarded by DARPA, and under LLNS subcontract B599861 awarded by DOE. The Government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to multiprocessor architecture, and more specifically to a ground-referenced single-ended signaling connected graphics processing unit multi-chip module.

BACKGROUND

A typical computing system includes a central processing unit (CPU), a graphics processing unit (GPU), a high-capacity memory subsystem, and set of interface subsystems. To achieve generational increases in system performance, sequential generations of GPU devices require increasing degrees of performance and integration. Conventional GPU devices typically achieve higher degrees of performance and integration by implementing an increasing number of graphics processing cluster (GPC) partitions and associated frame buffer (FB) partitions on a single die or “chip.” The GPC partitions are typically coupled to the FB partitions through a crossbar circuit. Cache memory may also be added to each chip.

Measures of die area for GPU devices have increased over time, as more GPC partitions and more FB partitions, each of increasing complexity, are integrated into a single GPU chip. One advantage of integrating multiple partitions and other subsystems onto a single die is that high-performance may be achieved by scaling conventional design techniques and leveraging advances in fabrication technology that enable greater circuit density.

However, one disadvantage of simply integrating more circuitry onto a single chip is that manufacturing cost for the chip typically increases disproportionately with respect to die area, increasing marginal cost associated with each additional GPC or FB. More specifically, manufacturing cost for a given chip is typically a strong function of die area for the chip. In many cases, die area associated with highly-integrated GPU devices is well above a characteristic cost knee, leading to disproportionate cost inefficiencies associated with fabricating advanced GPU chips.

Thus, there is a need for improving GPU architecture, and/or other issues associated with the prior art.

SUMMARY

A system of interconnected chips comprising a multi-chip module (MCM) is disclosed. The system includes a first processor chip, a graphics processing cluster (GPC) chip, and an MCM package configured to include the first processor chip, the GPC chip, and an interconnect circuit. The first processor chip is configured to include a first single-ended signaling interface circuit. A first set of electrical traces fabricated within the MCM package is configured to couple the first single-ended signaling interface circuit to the interconnect circuit. The GPC chip is configured to include a multi-threaded processor core configured to execute graphics shader programs and a second single-ended interface circuit. A second set of electrical traces fabricated within the MCM package is configured to couple the second single-ended signaling interface circuit to the interconnect circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a ground-referenced single-ended signaling (GRS) system that implements a GRS transmitter based on a flying capacitor charge pump, in accordance with one embodiment;

FIG. 1B illustrates operation of a data driver in a pre-charge state and in two different data-dependent drive states, in accordance with one embodiment;

FIG. 1C illustrates a GRS system that implements a GRS transmitter based on a dual-capacitor charge pump, in accordance with one embodiment;

FIG. 1D illustrates operation of a data driver in a pre-charge state, in accordance with one embodiment;

FIG. 1E illustrates operation of a data driver in different data-dependent drive states, in accordance with one embodiment;

FIG. 1F illustrates operation of a ground-referenced single-ended data driver based on a flying capacitor charge pump, in accordance with one embodiment;

FIG. 1G illustrates operation of a ground-referenced single-ended data driver based on a dual capacitor charge pump, in accordance with one embodiment;

FIG. 2A illustrates an exemplary ground-referenced single-ended receiver, in accordance with one embodiment;

FIG. 2B illustrates an exemplary ground-referenced single-ended receiver, configured to demultiplex incoming data, in accordance with one embodiment;

FIG. 3 illustrates an exemplary transceiver pair, configured to implement ground-referenced single-ended signaling, in accordance with one embodiment;

FIG. 4A illustrates a ground-referenced single-ended data driver comprising a CMOS circuit, in accordance with one embodiment;

FIG. 4B illustrates a ground-referenced single-ended data driver in a pre-charge state associated with driving a data value of zero, in accordance with one embodiment;

FIG. 4C illustrates a ground-referenced single-ended data driver in a pre-charge state associated with driving a data value of one, in accordance with one embodiment;

FIG. 4D illustrates a ground-referenced single-ended data driver in a drive state, in accordance with one embodiment;

FIG. 5A illustrates a ground-referenced single-ended transmitter comprising two instances of a ground-referenced single-ended data driver, in accordance with one embodiment;

FIG. 5B illustrates timing for a ground-referenced single-ended transmitter comprising two ground-referenced single-ended data drivers, in accordance with one embodiment;

FIG. 5C illustrates a flow chart of a method for generating a ground-referenced single-ended signal, in accordance with one embodiment;

FIG. 6A illustrates a multi-chip graphics processing unit implemented as a multi-chip module, in accordance with one embodiment;

FIG. 6B illustrates a directly-connected multi-chip graphics processing unit implemented as a multi-chip module, in accordance with one embodiment;

FIG. 6C illustrates a hub-connected multi-chip graphics processing unit implemented as a multi-chip module, in accordance with one embodiment;

FIG. 6D illustrates a network-connected multi-chip graphics processing unit implemented as a multi-chip module, in accordance with one embodiment;

FIG. 6E illustrates a network-connected multi-chip graphics processing unit with frame buffer-attached memory implemented as a multi-chip module, in accordance with one embodiment;

FIG. 6F depicts a cross-section of a multi-chip module configured to implement a chip stack comprising a frame buffer chip and a memory subsystem, in accordance with one embodiment;

FIG. 6G illustrates a multi-chip graphics processing unit configured to include a processing chip and a system functions chip within a multi-chip module, in accordance with one embodiment;

FIG. 6H illustrates a multi-chip graphics processing unit configured to accommodate an additional processing chip within a multi-chip module, in accordance with one embodiment; and

FIG. 7 illustrates an exemplary system in which the various architecture and/or functionality of the various previous embodiments may be implemented.

DETAILED DESCRIPTION

A technique is provided for high-speed, single-ended signaling between different chips comprising a system-on-package device. A ground-referenced driver transmits a pulse having a polarity determined by a corresponding logic state. The pulse traverses a signal path and is received by a ground-referenced amplifier, which amplifies the pulse for interpretation as a conventional logic signal. Sets of ground-referenced drivers and ground-referenced amplifiers implement high-speed interfaces configured to interconnect different chips comprising the system-on-package device. The high-speed communication enabled by ground-referenced signaling advantageously improves bandwidth between different chips within the system-on-package device, enabling higher performance and higher density systems than provided by conventional signaling techniques.

Embodiments of the present invention implement a system comprising a plurality of different processor chips, one or more memory chips, and feature-specific chips coupled to a multi-chip package. Interconnections between the different chips are routed through the multi-chip package. At least one of the interconnections is configured to implement a ground-referenced single-ended signaling (GRS) link, described below.

A GRS data driver implements a charge pump driver configured to transmit a ground-referenced pulse on an associated signal line. In one implementation, a pulse of positive charge indicates a logical one, while a pulse of negative charge indicates a logical zero. The charge pump driver eliminates simultaneous switching noise (SSN) commonly associated with single-ended signaling by forcing transient signal current and ground current to be locally balanced, and by drawing a constant amount of charge from the power supply each half clock cycle, independent of the data being transmitted. The pulse is received and amplified by a common gate amplifier stage configured to use a local ground signal as an input reference. This configuration provides substantial immunity to common mode noise, the dominant source of transmission errors in single-ended signaling. A second amplifier stage translates a given received pulse to full-swing logic voltages, allowing the received pulse to be properly interpreted as one or two logic states by conventional logic circuitry. In one embodiment, a GRS receiver comprises a common gate amplifier stage, the second amplifier stage, and two storage elements, such as flip-flips, configured to capture received data during alternate clock phases.

A GRS transceiver includes a GRS data driver and a GRS receiver. The GRS transceiver transmits outbound data through the GRS data driver and receives inbound data through the GRS receiver. An isochronous GRS transceiver may also transmit clocking information having a fixed phase relationship to the outbound data and receives clocking information having a fixed phase relationship to the inbound data. A GRS interconnect includes two different GRS transceivers, coupled through an electrical trace that is manufactured within a common multi-chip module package.

FIG. 1A illustrates a ground-referenced single-ended signaling (GRS) system 100 that implements a GRS transmitter 110 based on a flying capacitor charge pump, in accordance with one embodiment. GRS system 100 includes GRS transmitter 110, a transmission path comprising a signal line 105 and a ground network 107, and a GRS receiver 130. In one embodiment, GRS transmitter 110 comprises two data drivers 112, 114. Input data signals D0 and D1 are presented to GRS transmitter 110 based on a clock signal CLK. Data driver 112 is configured to capture a logic state associated with input D0 and drive output signal Vout 116 onto signal line 105 with a pulse corresponding to the logic state of input D0 while CLK is low. Similarly, data driver 114 is configured to capture a logic state associated with input D1 and drive output signal Vout 116 onto signal line 105 with a pulse corresponding to the logic state of D1 while CLK is high. A sequence of pulses is formed along signal line 105 corresponding to a sequence of input data from inputs D0 and D1. The sequence of pulses is referenced to ground with a voltage swing that may be lower than conventional logic voltage swings. GRS receiver 130 is configured to amplify an incoming sequence of pulses from signal line 105 and translate the pulses to a conventional logic voltage swing so the pulses may be properly interpreted as logic signals on amplifier output signal 132. For example, the sequence of pulses along signal line 105 may have a nominal amplitude of plus or minus one-hundred millivolts, while amplifier output signal 132 may have a corresponding voltage swing of twelve hundred millivolts to zero volts with respect to ground if logic coupled to amplifier output signal 132 operates on a twelve hundred millivolt positive supply rail.

In one embodiment, GRS transmitter 110 is fabricated on a transmitter chip and GRS receiver 130 is fabricated on a receiver chip distinct from the transmitter chip. Pads 120 comprise bonding pads configured to couple output signal Vout 116 from the transmitter chip to signal line 105, which is fabricated as an impedance-controlled trace within a multi-chip module (MCM) package 190. Pads 122 comprise bonding pads configured to couple a local ground signal within the transmitter chip to ground network 107, fabricated within MCM package 190. Similarly, pads 124 comprise bonding pads configured to couple signal line 105 to an input signal for GRS receiver 130 within the receiver chip, and pads 126 comprise bonding pads configured to couple ground network 107 to a local ground within the receiver chip. A termination resistor RTx is coupled between output signal Vout 116 and the local ground within the transmitter chip to absorb incoming signals, such as reflections or induced noise signals. A termination resistor RRx is coupled across inputs to GRS receiver 130 to similarly absorb incoming signals at the receiver chip.

Data driver 112 comprises capacitor C0, and switches S01 through S06. Switch S01 enables a first node of capacitor C0 to be coupled to a positive supply rail, while switch S02 enables a second node of capacitor C0 to be coupled to a local ground net. Switches S01 and S02 are active (closed) during a pre-charge state for data driver 112, defined when CLK is equal to a logical “1” value. Switch 803 enables the first node of capacitor C0 to be coupled to GND, while switch S06 enables the second node of capacitor C0 to be coupled to GND. Switch S04 enables the first node of capacitor C0 to be coupled to Vout 116, while switch S05 enables the second node of capacitor C0 to be coupled to Vout 116. When CLK is equal to a logical “0” value, switches S04 and S06 are active when data driver 112 is driving a logical “1” value to Vout 116, or switches S03 and S05 are active when data driver 112 is driving a logical “0” value to Vout 116. Data driver 114 comprises a substantially identical circuit topology, with an inverted sense for CLK, so that data driver 114 is in a pre-charge state when CLK is equal to a logical “0” value and driving Vout 116 when CLK is equal to a logical “1” value.

In one embodiment, switches S01 through S06 and switches S11 through S16 are fabricated using monolithic complementary metal-oxide semiconductor (CMOS) devices, such as enhancement mode n-channel and p-channel field-effect transistors. Any technically feasible logic circuit topologies may be implemented to drive switches S01-S06 and switches S11-S16 into individually active or inactive states without departing the scope and spirit of embodiments of the present invention.

FIG. 1B illustrates operation of a data driver 112 in a pre-charge state and in two different data-dependent drive states, in accordance with one embodiment. As shown, when CLK is equal to a logical “1” value, data driver 112 is in a pre-charge state, whereby switches S01 and S02 are active and capacitor C0 charges to a voltage corresponding approximately to a positive supply rail, such as a “VDD” supply rail. All of switches S03-S06 are inactive (open) during the pre-charge state. When CLK is equal to a logical “0” value, two of switches S03-S06 are configured to couple capacitor C0 to Vout 116 to transmit a pulse having a polarity corresponding to a logical value for D0. To drive a logical “0” value, switches S03 and S05 are driven active, thereby coupling a negative charge relative to ground onto Vout 116. To drive a logical “1” value, switches S04 and S06 are driven active, thereby coupling a positive charge relative to ground onto Vout 116.

FIG. 1C illustrates a GRS system 102 that implements a GRS transmitter 150 based on a dual-capacitor charge pump, in accordance with one embodiment. GRS system 102 includes GRS transmitter 150, a transmission path comprising a signal line 105 and a ground network 107, and a GRS receiver 130. In one embodiment, GRS transmitter 150 comprises two data drivers 152 and 154. Operation of GRS system 102 is substantially identical to the operation of GRS system 100 described above in FIGS. 1A and 1B, with the exception of the internal topology and operation of data drivers 152 and 154.

Data driver 152 comprises capacitors C0A and C0B, as well as switches S0A through S0H. Switch S0A enables a first node of capacitor C0A to be coupled to a positive supply rail, while switch S0C enables the first node to be coupled to a local ground net. Switch S0B enables a second node of capacitor C0A to be coupled to Vout 116, while switch S0D enables the second node to be coupled to the local ground net. Similarly, switch S0E enables a first node of capacitor C0B to be coupled to the positive supply rail, while switch S0G enables the first node to be coupled to the local ground net. Switch S0F enables a second node of capacitor C0B to be coupled to Vout 116, while switch S0H enables the second node to be coupled to the local ground net.

A pre-charge state for data driver 152 is defined when CLK is equal to a logical “1” value. During the pre-charge state, switches S0A, S0D, S0G, and S0H are driven active, pre-charging capacitor C0A to a voltage corresponding to the positive supply rail relative to the local ground net, and pre-charging capacitor C0B to have approximately no charge. When CLK is equal to a logical “0” value, either capacitor C0A is coupled to Vout 116 to generate a negative pulse or capacitor C0B is coupled to Vout 116 to generate a positive pulse, as described below in conjunction with FIG. 1E. Data driver 154 comprises a substantially identical circuit topology, with an inverted sense for CLK, so that data driver 154 is in a pre-charge state when CLK is equal to a logical “0” value and driving Vout 116 when CLK is equal to a logical “1” value.

In one embodiment, switches S0A through S0H and switches S1A through S1H are fabricated using monolithic CMOS devices, such as enhancement mode n-channel and p-channel FETs. Any technically feasible logic circuit topologies may be implemented to drive switches S0A-S0H and switches S1A-S1H into individually active or inactive states without departing the scope and spirit of embodiments of the present invention.

FIG. 1D illustrates operation of data driver 152 in a pre-charge state, in accordance with one embodiment. As shown, when CLK is equal to a logical “1” value, switch S0A is active, coupling a first node of capacitor C0A to a positive supply rail, and switch S0D is active, coupling a second node of capacitor C0A to a local ground net. At the same time, switch S0G is active, coupling a first node of capacitor C0B to ground, and switch S0H is active, coupling a second node of capacitor C0B to ground. By the end of this pre-charge state, capacitor C0B is substantially discharged.

FIG. 1E illustrates operation of data driver 152 in different data-dependent drive states, in accordance with one embodiment. As shown, when CLK is equal to a logical “0” value and D0 is equal to a logical “0” value, switches S0C and S0B are configured to couple capacitor C0A to Vout 116 to transmit a pulse having a negative polarity. Alternatively, when CLK is equal to a logical “0” value and D0 is equal to a logical “1” value, switches S0E and S0F are configured to couple capacitor C0B to Vout 116 to transmit a pulse having a positive polarity. Here, the positive supply rail is assumed to have adequate high-frequency capacitive coupling to the local ground net to force transient return current through the local ground net in conjunction with driving Vout 116 with a positive pulse.

More illustrative information will now be set forth regarding various optional architectures and features with which the foregoing framework may or may not be implemented, per the desires of a designer or user. It should be strongly noted that the following information is set forth for illustrative purposes and should not be construed as limiting in any manner. Any of the following features may be optionally incorporated with or without the exclusion of other features described.

FIG. 1F illustrates operation of a ground-referenced single-ended data driver 162 based on a flying capacitor charge pump, in accordance with one embodiment. One or more instances of data driver 162 may be configured to operate as data drivers within a GRS transmitter. For example, an instance of data driver 162 may be configured to operate in place of data driver 112 within GRS transmitter 110 of FIG. 1A. Similarly, an instance of data driver 162 may be configured to operate in place of data driver 114.

Data driver 162 includes capacitor C2, and switches S20, S21, S22, S23, and S24, configured to pre-charge capacitor C2 during a pre-charge phase, and discharge capacitor C2 into Vout 116 during a data output phase. In one embodiment, a first instance of data driver 162 is configured to operate in a pre-charge phase when a clock signal is in a logical “0” state and a data output phase when the clock signal is in a logical “1” state. A second instance of data driver 162 is configured to operate in a pre-charge phase when the clock signal is in a logical “1” state and a data output phase when the clock signal is in a logical “0” state.

When each instance of data driver 162 is in the pre-charge phase, if D0 is in a logical “1” state, then switches S22 and S21 are active, while switches S20, S23, and S24 are inactive. While in the pre-charge phase, if D0 is in a logical “0” state, then switches S20 and S23 are active, while switches S21, S22, and S24 are inactive. During a data output phase, switches S21 and S24 are active, while switches S20, S22, and S23 are inactive. In sum, flying capacitor C2 is pre-charged with either a positive or negative polarity charge during the pre-charge phase. The charge is then discharged through ground and Vout 116 during the data output phase.

FIG. 1G illustrates operation of a ground-referenced single-ended data driver 172 based on a dual capacitor charge pump, in accordance with one embodiment. One or more instances of data driver 172 may be configured to operate as data drivers within a GRS transmitter. For example, an instance of data driver 172 may be configured to operate in place of data driver 112 within GRS transmitter 110 of FIG. 1A. Similarly, an instance of data driver 162 may be configured to operate in place of data driver 114.

Data driver 172 includes capacitors C3, C4, and switches S30, S31, S32, S33, S40, S41, and S42, configured to pre-charge capacitors C3 and C4 during a pre-charge phase, and discharge one of capacitors C3, C4 into Vout 116 during a data output phase. In one embodiment, a first instance of data driver 172 is configured to operate in a pre-charge phase when a clock signal is in a logical “0” state and a data output phase when the clock signal is in a logical “1” state. A second instance of data driver 172 is configured to operate in a pre-charge phase when the clock signal is in a logical “1” state and a data output phase when the clock signal is in a logical “0” state.

When each instance of data driver 172 is in the pre-charge phase, switches S30, S33, S40, and S41 are active, and switches S31, S32, and S42 are inactive. During the data output phase, if D0 is in a logical “0” state, then switches S31 and S32 are active, allowing capacitor C3 to discharge a negative polarity charge into Vout 116. At the same time, switches S30, S33, and S40-S42 are inactive. During the data output phase, if D0 is in a logical “1” state, then switches S41 and S42 are active, allowing capacitor C4 to discharge a positive polarity charge into Vout 116. At the same time, switches S40 and S30-S33 are inactive.

FIG. 2A illustrates an exemplary GRS receiver 130, in accordance with one embodiment. As shown, GRS receiver 130 receives input signals Vin 264 and GRef 266, and generates amplifier output signal 132. In one embodiment, an arriving pulse at Vin 264 having a positive voltage with respect to GRef 266 represents a logical “1” and an arriving pulse at Vin 264 having a negative voltage with respect to GRef 266 represents a logical “0”. GRS receiver 130 amplifies a differential voltage between input signals Vin 264 and GRef 266 to generate a corresponding difference signal 262. In one embodiment, GRS receiver 130 is designed to bias difference signal 262 to be centered about a switching threshold for inverter inv3, which amplifies difference signal 262 to generate amplifier output signal 132 according to conventional logic voltage levels.

In one embodiment, GRS receiver 130 comprises resistors R1 through R4, inverters inv1 through inv3, capacitor C5, and field-effect transistors n1 and n2. Resistors R2 and R4 may be implemented as variable resistors, using any technically feasible technique. One exemplary implementation of a variable resistor provides digital control of a resistance value and comprises a set of n-channel FETs connected in a parallel configuration. Each n-channel FET is controlled by a different digital control signal from a control word used to establish the resistance value. If the control word is defined to be a binary number, a corresponding resistance value for the set of n-channel FETs may be monotonic if the n-channel FETs are sized appropriately. In a practical implementation, resistors R2 and R4 are tuned to balance the termination of incoming pulses and current injected into Vin 264 and GRef 266 by GRS receiver 130. A monotonic mapping from a binary code word to a resistance value simplifies any required digital trimming needed to achieve balanced termination. Any technically feasible technique may be implemented to adjust resistors R2 and R4 to achieve balanced termination.

Resistors R1 and R3 may also be implemented using any technically feasible technique. For example, resistors R1 and R3 may be implemented as p-channel FETs that are biased appropriately. Inverters inv1 and inv2 provide gain, while capacitor C5 serves to stabilize a loop formed by inverters inv1 and inv2, in conjunction with resistor R1 and FET n1.

FIG. 2B illustrates an exemplary GRS receiver unit 270, configured to demultiplex incoming data, in accordance with one embodiment. GRS receiver unit 270 comprises a GRS receiver 130, and storage elements configured to capture and store the logic state of amplifier output signal 132 on alternating clock phases to demultiplex input data represented as arriving pulses on input signal Vin 264, referenced to input signal GRef 266. Each output signal D0 284 and D1 282 presents captured input data at half the frequency of the arriving data pulses.

In one embodiment, the storage elements comprise a positive edge triggered flip-flop 274 and a negative edge triggered flip-flop 272. As shown, positive edge triggered flip-flop 274 is configured to capture D0 during the rising edge of a clock signal CLK 268, while negative edge triggered flip-flop 272 is configured to capture D1 during a falling edge of CLK 268. Such a configuration assumes that CLK 268 and amplifier output signal 132 transition together and that flip-flops 272 and 274 require more setup time than hold time. In alternative embodiments, D0 is captured on a falling edge of CLK 268, while D1 is captured on a rising edge of CLK 268. In other alternative embodiments, the storage elements comprise level-sensitive latches rather than flip-flops.

FIG. 3 illustrates an exemplary transceiver pair 300, configured to implement GRS signaling, in accordance with one embodiment. As shown, the transceiver pair 300 includes transceiver unit 310 coupled to transceiver unit 370 through signal lines 352, 354, 356, and 358. Signal lines 352, 354, 356, and 358 may be manufactured as controlled-impedance traces embedded within an MCM package 190. Transceiver 310 is configured to receive a reference clock 312 operating at one half the data transmission rate for the signal lines. Adjustable phase delay 332 may introduce an adjustable phase delay prior to transmitting reference clock 312 to GRS transmitter 322, GRS transmitter 324, and serializer 334.

As shown, the GRS transmitter 322 is configured to transmit a sequential “01” pattern to the GRS receiver 382 through pads 342, signal line 352, and pads 362. In one embodiment, this “01” pattern is transmitted at substantially the same phase as data transmitted from the GRS transmitter 324 to GRS receiver 384 through pads 344, signal line 354, and pads 364. Serializer 334 receives transmit data 314 at a lower frequency than reference clock 312, but at a correspondingly wider parallel width. For example, if reference clock 312 is configured to operate at 10 GHz, and serializer 334 is configured to multiplex a sixteen bit word into two bits for transmission through GRS transmitter 324, then sixteen bit words may arrive at a rate of 10 GHz divided by eight or 1.25 GHz. Here, a transmission data clock 313 may be generated by serializer 334 to operate at 1.25 GHz for timing transfers of arriving transmit data 314. In this example, reference clock 312 has a 100 pS period and each distinct bit transmitted by GRS transmitters 322 and 324 has a unit interval of 50 pS.

GRS receiver 382 receives a phase-delayed version of reference clock 312 through signal line 352 and generates a local reference clock 383, which may be coupled to GRS receiver 384 for capturing arriving pulses on signal line 354. Local reference clock 383 may also be coupled to deserializer 394 for capturing and demultiplexing data from GRS receiver 384. Extending the above example, GRS receiver 384 may capture arriving pulses on alternating clock phases of local reference clock 383, operating at 10 GHz, to generate two bits every 100 pS. Deserializer 394 is configured to demultiplex sequential data comprising two bits from GRS receiver 384 and to generate corresponding sixteen-bit words at a rate of 1.25 GHz. The sixteen-bit words are presented as receive data 374. Deserializer 394 may generate receiver data clock 373 to reflect appropriate clocking for receive data 374. Receive data 374 represents a local copy of transmit data 314. In one embodiment, deserializer 394 is configured to align arriving data along word boundaries. Persons skilled in the art will understand that serialization and deserialization of parallel data may require alignment of the parallel data along word boundaries and that well-known techniques in the art may be implemented by transceiver unit 370 or associated logic without departing the scope and spirit of embodiments of the present invention.

Serializes 396 captures arriving transmit data 376 and serializes the data for transmission by GRS transmitter 386 through signal line 356. In one embodiment, serializer 396 generates transmit data clock 375 based on local reference clock 383 as a clocking reference for arriving transmit data 376. GRS receiver 326 captures the data arriving from signal line 356 and deserializer 336 demultiplexes the data into words, presented as receive data 316. GRS transmitter 388 is configured to transmit a sequential “01” pattern to GRS receiver 328 through pads 368, signal line 358, and pads 348. In one embodiment, this “01” pattern is transmitted at substantially the same phase as data transmitted from GRS transmitter 386 to GRS receiver 326 through pads 366, signal line 356, and pads 346. GRS receiver 328 and adjustable phase delay 338 generate receive clock 318 based on the sequential “01” pattern. In one embodiment, receive data clock 315 is generated by deserializer 336 to reflect appropriate clocking for receive data 316.

Determining a proper phase delay value for adjustable phase delay 332 and adjustable phase delay 338 may be performed using any technically feasible technique. For example, phase delay values for adjustable phase delay 332 and adjustable phase delay 338 may be swept over a range of phase delay values during a link training phase, whereby phase delays corresponding to a substantially minimum bit error rate during training are determined and used for normal link operation.

Although an isochronous clocking model is illustrated herein for transmitting data between transceiver unit 310 and transceiver unit 370, any technically feasible clocking model may be implemented without departing the scope and spirit of embodiments of the present invention.

FIG. 4A illustrates a GRS data driver 400 comprising a CMOS circuit, in accordance with one embodiment. As shown, the CMOS circuit illustrates a circuit topology that may be used to implement the data driver 162 of FIG. 1F using CMOS circuit elements. Specifically, switches S20 and S22 are implemented as p-channel FET p40, and p-channel FET p42, respectively; and switches S21, S23, and S24 are implemented as n-channel FET n41, n-channel FET n43, and n-channel FET n44, respectively. A reference node 410 is coupled to a capacitor C7, p-channel FET p40 and n-channel FET n41. An output node 412 is coupled to an opposing side of capacitor C7, as well as to p-channel FET p42, n-channel FET n43, and n-channel FET n44.

Control signal g40 is coupled to a gate node of p-channel FET p40. When control signal g40 is driven to a logical 0 level, p-channel FET p40 turns on, pulling node 410 to a voltage level associated with VDD. Control signal g41 is coupled to a gate node of n-channel FET n41. When control signal g41 is driven to a logical 1 level, n-channel FET n41 turns on, pulling node 410 to a voltage level associated with GND. Similarly, p-channel FET p42 responds to control signal g42, selectively pulling node 412 to VDD, while n-channel FET n43 responds to control signal g43, selectively pulling node 412 to GND. Control signal g44 is coupled to a gate node of n-channel FET n44. When control signal g44 is driven to a logical 0 level, n-channel FET n44 substantially isolates node 412 from node Vout 416. However, when control signal g44 is driven to a logical 1 level, n-channel FET n44 forms a low impedance path between node 412 and Vout 416. As described below in conjunction with FIG. 4D, this low impedance path facilitates driving Vout 416 with an appropriate signal.

GRS data driver 400 operates primarily in three different states, including a first pre-charge state for subsequently driving a data value of zero, a second pre-charge state for subsequently driving a data value of one, and a drive state for driving a signal line, such as signal line 105, with a signal corresponding to a preceding pre-charge state. These states are illustrated below in FIGS. 4B-4D. Transitions between pre-charge states and the drive state are orchestrated by control signals g40 through g44.

FIG. 4B illustrates GRS data driver 400 in the first pre-charge state that is associated with driving a data value of zero, in accordance with one embodiment. As shown, in the first pre-charge state, control signal g40 is set to zero, to turn on p-channel FET p40, thereby coupling node 410 to VDD. At the same time, control signal g43 is set to one (1), to turn on n-channel FET n43, thereby coupling node 412 to GND. Also, control signal g42 is set to one to turn off p-channel FET p42, and control signals g41 and g44 are set to zero to turn off n-channel FET n41 and n-channel FET n44, respectively. In this first pre-charge state, capacitor C7 is charged with a positive charge on node 410 and a negative charge on node 412, which is electrically isolated from node Vout 416.

FIG. 4C illustrates GRS data driver 400 in the second pre-charge state that is associated with driving a data value of one, in accordance with one embodiment. As shown, in the second pre-charge state, control signal g42 is set to zero, to turn on p-channel FET p42, thereby coupling node 412 to VDD. At the same time, control signal g41 is set to one, to turn on n-channel FET n41, thereby coupling node 410 to GND. Also, control signal g40 is set to one to turn off p-channel FET p40, and control signals g43 and g44 are set to zero to turn off n-channel FET n43 and n-channel FET n44, respectively. In this second pre-charge state, capacitor C7 is charged with a negative charge on node 410 and a positive charge on node 412, which is electrically isolated from node Vout 416.

FIG. 4D illustrates GRS data driver 400 in a drive state, in accordance with one embodiment. As shown, control signal g41 is set to one, coupling node 410 to GND and control signal g44 is set to one, coupling node 412 to node Vout 416. Control signals g40 and g42 are set to one, to turn off p-channel FET p40 and p-channel FET p42, respectively. Additionally, control signal g43 is set to zero, to turn off n-channel FET n43. In this state, capacitor C7 discharges into node Vout 416. If a negative charge has been accumulated in capacitor C7 in a previous pre-charge state, then C7 discharges the negative charge into node Vout 416 with respect to GND. Otherwise, if a positive charge has been accumulated in capacitor C7 in a previous pre-charge state, then C7 discharges a positive charge into node Vout 416 with respect to GND. Current passing through node Vout 416 is substantially balanced with a corresponding ground current passing through GND.

Capacitor C7 may be implemented using any technically feasible technique without departing the scope and spirit of embodiments of the present invention. In one embodiment, the capacitor C7 is implemented using n-channel FETs. For example, a gate node of a first n-channel FET may be coupled to node 412 of FIG. 4A to form a back-to-back metal-oxide transistor capacitor. Additionally, source and drain nodes of the first n-channel FET may be coupled to node 410. A gate node of a second n-channel FET may be coupled to node 410, while source and drain nodes of the second n-channel FET may be coupled to node 412. Gate capacitance is relatively area-efficient compared to other capacitor structures available within a CMOS process. However, gate capacitance varies significantly with charge polarity. To compensate for polarity-dependent gate capacitance, two n-channel devices are symmetrically configured to store charge in opposite polarities. In this way, a positive pulse discharged into node Vout 416 has a substantially equal magnitude relative to a negative pulse discharged into Vout 416.

In another embodiment, the capacitor C7 may be implemented using traces in adjacent metal layers. For example, traces in sequential metal layers may be configured to provide plate capacitance (Cp) and edge capacitance (Ce) between nodes 410 and 412. Unlike gate capacitance, plate and edge capacitance between metal structures embedded within conventional dielectric materials are stable with respect to polarity. However, a capacitor formed using metal layer traces may require more die area compared to a capacitor formed using gate capacitance for an equivalent capacitance value. While two parallel traces on two adjacent layers may be used to implement the capacitor C7, one skilled in the art will understand that such a metal-oxide-metal (MOM) capacitor can be realized using more than two layers and more than two adjacent traces on each layer.

FIG. 5A illustrates a GRS transmitter 550 comprising two instances of a GRS data driver 400, in accordance with one embodiment. As shown, GRS transmitter 550 receives data input signals D0 and D1 that are synchronized to clock signal CLK. Control logic 502 receives signals D0, D1 and CLK, and, in response, generates driver control signals 510 and driver control signals 512. In one embodiment, driver control signals 510 comprise control signals g40 through g44 for instance 400(0) of GRS data driver 400, and driver control signals 512 comprise control signals g4 through g44 for instance 400(1) of GRS data driver 400.

In one embodiment, when CLK is in a logical one state, control logic 502 configures instance 400(0) to operate in a pre-charge state. If D0 is in a logical zero state, then instance 400(0) enters the pre-charge state associated with driving a data value of zero, illustrated previously in FIG. 4B. Here, driver control signals 510 are generated such that g40=0, g41=0, g42=1, g43=1, and g44=0. If, instead, D0 is in a logical one state, then instance 400(0) enters the pre-charge state associated with driving a data value of one, illustrated previously in FIG. 4C. Here, driver control signals 510 are generated such that g40=1, g41=1, g42=0, g43=0, and g44=0. When CLK is in a logical zero state, control logic 502 configures instance 400(0) to operate in the drive state, illustrated previously in FIG. 4D. Here, driver control signals 510 are generated such that g40=1, g41=1, g42=1, g43=0, and g44=1.

When CLK is in a logical zero state, control logic 502 configures instance 400(1) to operate in a pre-charge state. If D1 is in a logical zero state, then instance 400(1) enters the pre-charge state associated with driving a data value of zero, illustrated previously in FIG. 4B. Here, driver control signals 512 are generated such that g40=0, g41=0, g42=1, g43=1, and g44=0. If, instead, D1 is in a logical one state, then instance 400(1) enters the pre-charge state associated with driving a data value of one, illustrated previously in FIG. 4C. Here, driver control signals 512 are generated such that g40=1, g41=1, g42=0, g43=0, and g44=0. When CLK is in a logical one state, control logic 502 configures instance 400(1) to operate in the drive state, illustrated previously in FIG. 4D. Here, driver control signals 510 are generated such that g40=1, g41=1, g42=1, g43=0, and g44=1.

Each instance 400(0), 400(1) is coupled to a common Vout 416 signal, which is further coupled to a pad 520. In one embodiment, Vout 416 is coupled to pad 522 through resistor RTx. Pad 522 is coupled to a circuit ground node, corresponding to GND in FIGS. 4A-4D.

In one embodiment, GRS transmitter 550 is configured to replace GRS transmitter 110 of FIG. 1A. Here, pad 520 couples Vout 416 to signal line 105 and pad 522 couples GND to ground network 107. In such a configuration, GRS receiver 130 receives data from GRS transmitter 550. In certain embodiments, GRS transmitter 550 comprises GRS Tx 322, GRS Tx 324, GRS Tx 386, and GRS Tx 388 of FIG. 3.

FIG. 5B illustrates timing for a GRS transmitter 550, in accordance with one embodiment. As shown, one bit of data from input D0 is transmitted to Vout 416 during time k+1 when CLK is in a logical zero state, and one bit of data from input D1 is transmitted to Vout 416 during time k+2 when CLK is in a logical one state. In one embodiment, inputs D0 and D1 are synchronous to and are updated on the rising edge of CLK. In such an embodiment, instance 400(1) is in a data driving state when inputs D0 and D1 change in response to a rising edge of CLK going into time k. On the rising edge of CLK going into time k, instance 400(0) enters a pre-charge state, thereby sampling data on D0. On the falling edge of CLK exiting time k+1 and entering time k+2, instance 400(0) enters a data driving state and drives the captured data from D0 onto Vout 416. On the falling edge of CLK going into time k+1, instance 400(1) enters a pre-charge state, thereby sampling data on D1. On the rising edge of CLK exiting time k+1 and entering time k+2, instance 400(1) enters a data driving state and drives the captured data from D1 onto Vout 416. In this way, data comprising D0 and D1 may be presented to GRS transmitter 550 using conventional logic having conventional single-edge synchronous timing, while GRS transmitter 550 time-multiplexes the data for transmission at a double data rate. In other words, two data transfers occur in each period or cycle of the CLK. In a preferred embodiment, D0 is latched when CLK is low to ensure that D0 is stable while being used to control the pre-charge of instance 400(0). Similarly, D1 is latched when CLK is high to ensure D1 is stable while being used to control the pre-charge of instance 400(1).

In other embodiments, a GRS transmitter comprising more than two instances of GRS data driver 400 is configured to receive a data bit per instance of GRS data driver 400 and to time-multiplex the data at a correspondingly higher data rate. In such embodiments, multiple clock signals may be required to provide appropriate timing for pre-charging and driving data to time-multiplex the data.

FIG. 5C illustrates a flow chart of a method 560 for generating a ground-referenced single-ended signal, in accordance with one embodiment. Although method 560 is described in conjunction with FIGS. 4A-5B to implement a two to one time-multiplexing ratio of input data to output data, persons of ordinary skill in the art will understand that any system that performs method 560 is within the scope and spirit of embodiments of the present invention.

Method 560 begins in step 565, where a first data driver, such as Instance 400(0) of GRS data driver 400, samples a first bit of data by pre-charging a first capacitor during a first time k. The first capacitor is charged to have a polarity corresponding to a logic level for the first bit of data. In step 570, a second data driver, such as instance 400(1) of GRS data driver 400, samples a second bit of data by pre-charging a second capacitor during a time k+1. The second capacitor is charged to have a polarity corresponding to a logic level for the second bit of data.

In step 575, the first data driver drives an output signal, such as Vout 416 of FIGS. 4A-4D or Vout 416 of FIG. 5A, to reflect the first bit of data by coupling the first capacitor to the output signal during the time k+1. Here, the first capacitor is coupled between a ground network and the output signal. The polarity of charge on the first capacitor was established in step 565, based on the logic level for the first bit of data. When coupled to the output signal, the first capacitor therefore reflects the logic level for the first bit of data.

In step 580, the second data driver drives the output signal to reflect the second bit of data by coupling the second capacitor to the output signal during a time k+2. Here, the second capacitor is coupled between a ground network and the output signal. The polarity of charge on the second capacitor was established in step 570, based on the logic level for the second bit of data. When coupled to the output signal, the second capacitor therefore reflects the logic level for the second bit of data. Method 560 terminates after driving the output signal to reflect the second bit of data.

In other embodiments, a time-multiplexing ratio of greater than two may be implemented and at least one additional phase-related clock may be provided to orchestrate operation of more than three instances of GRS data driver 400.

Multi-Chip GPU System with Ground-Referenced Signaling

FIG. 6A illustrates a multi-chip graphics processing unit (GPU) implemented as a multi-chip module (MCM) 600, in accordance with one embodiment. As shown, MCM 600 comprises an MCM package 190, two or more graphics processing cluster (GPC) chips 610, and an interconnect 614, configured to facilitate communication between and among GPC chips 610, and between and among frame buffer (FB) chips 620.

Each GPC chip 610 may include one or more multi-threaded graphics processing cores, configured to execute one or more thread programs. The thread programs may implement, without limitation, geometric operations, pixel operations, general-purpose computation operations, or any combination thereof. The thread programs may comprise graphics shader programs. Each GPC chip 610 may also include cache memory for each graphics processing core, as well as cache memory shared by two or more graphics processing cores. For example, each GPC chip 610 may include a first level cache associated with each graphics processing core. Each GPC chip 610 may also include a second level cache shared among one or more graphics processing cores included within GPC chip 610. In certain embodiments, GPC chip 610(0) is configured to provide high computational performance, GPC chip 610(1) is configured to operate with low power consumption. In such embodiments, GPC chip 610(0) may be fabricated from a high-performance fabrication technology, while GPC chip 610(1) may be fabricated from a low-power fabrication technology. In certain embodiments, GPC chip 610(0) is designed for relatively high performance, while GPC chip 610(1) is designed using the same fabrication technology for relatively low power. In one embodiment, GPC chip 610(0) includes a larger number of graphics processing cores than GPC chip 610(1). The GPC chips 610(2) and 610(3) may each be designed for high performance or for low power consumption. In one embodiment, system functions chip 618 is fabricated using a different fabrication technology than GPC chips 610 or FB chips 620. For example, system functions chip 618 may be fabricated using a previous generation of fabrication technology, while GPC chips 610 may be fabricated using a more advanced and higher-performance fabrication technology.

Each GPC chip 610 is coupled to an interconnect 614 through a corresponding interconnect link 612. As illustrated in greater detail in FIGS. 6B-6E, interconnect 614 may implement different topologies that facilitate communication among GPC chips 610 and FB chips 620. In one embodiment, each interconnect link 612 comprises one or more GRS transceivers disposed within a corresponding GPC chip 610 and associated electrical traces manufactured within MCM package 190. Each GRS transceiver may include one or more bidirectional or one or more unidirectional data signals, according to implementation-specific requirements.

In one embodiment, each transceiver is configured to multiplex data from core circuitry in a ratio of sixteen to one, so that each GRS signal line transmits sixteen bits in the time needed for one bit per signal to be transmitted at a core circuitry operating frequency. In such an embodiment, a two hundred and fifty-six bit on-chip data bus may be multiplexed into sixteen GRS signal lines. In certain embodiments, one six teen-signal GRS channel is configured to transmit data from a GPC chip 610, and another sixteen-signal GRS channel is configured to transmit data to the GPC chip 610.

Each FB chip 620 is coupled to interconnect 614 through a corresponding interconnect link 622. Each FB chip 620 is also coupled to a memory subsystem (not shown) through a corresponding memory link 624. A given FB chip 620 may implement, without limitation, pixel blending and compositing operations. Different blend modes may be implemented and the FB chip 620 may perform one or more of the different blend modes on pixel data transmitted from the GPCs 610 to the memory sub-system, according to specific algorithmic requirements for the data. In summary, the FB chip 620 implements a raster operations unit, including read-modify-write operations to a frame buffer memory. Each FB chip 620 may include a cache, such as a layer two cache. More generally, each FB chip 620 implements a memory controller and may operate as a memory controller on behalf of other system elements, such as GPCs 610.

Any technically feasible communication protocol may be implemented for transmitting data over interconnect links 612. In one embodiment, the communication protocol specifies, without limitation, a memory read request that includes an access address, a read response (acknowledgement) that includes requested read data, a memory write request that includes an access address and write data, and a write acknowledgement that indicates a successful write operation. In certain embodiments, the read request and the write request also include an access length specified in bytes, words, or any technically feasible measure of data length. In one embodiment, a given access request comprises a split transaction. In an alternative embodiment, a given access request comprises a blocking transaction. In certain embodiments, the communication protocol specifies a message passing mechanism for transmitting data packets to a destination device. In one embodiment, the communication protocol, implemented for transmitting data through interconnect links 612, specifies a cache coherence protocol. The cache coherence protocol may provide a broadcast mechanism for maintaining cache coherence, a directory-based mechanism for maintaining cache coherence, or any technically feasible mechanism for maintaining cache coherence among two or more caches or memory subsystems without departing the scope and spirit of embodiments of the present invention. In one embodiment, the cache coherence protocol implements an invalidation mechanism for processing cache writes. Alternatively, the cache coherence protocol implements an update mechanism for processing cache writes. In one embodiment, the cache coherence protocol implements a write-through mechanism for processing certain writes.

MCM 600 may also include one or more memory subsystems (not shown), coupled to FB chips 620, either directly or through an additional interconnect (not shown). In one embodiment, each memory subsystem comprises a DRAM chip. In another embodiment, each memory subsystem comprises a cache memory chip. The cache memory chip may comprise a second level cache, a third level cache, a cache slice, or any other technically feasible cache memory element. In yet another embodiment, each memory subsystem comprises a stack of memory chips including at least one DRAM chip, or at least one cache memory chip, or a combination thereof. In still other embodiments, each memory subsystem comprises an interface shim chip and at least one DRAM chip, at least one cache memory chip, or at least one DRAM chip and at least one cache memory chip. The interface shim chip may include a memory controller, configured to receive access requests (commands), and process the access requests by generating further access requests that directly target DRAM chips or cache memory chips coupled to the shim chip. In certain embodiments, each memory subsystem is configured to communicate through a GRS transceiver comprising one or more data signals to at least one FB chip 620. In such embodiments, a given memory subsystem may be coupled directly to the at least one FB chip 620, or the memory subsystem may be coupled indirectly through interconnect 614, or the additional interconnect.

In one embodiment, illustrated below in FIG. 6F, a given FB chip 620 is included within a vertical stack of chips that includes a corresponding memory subsystem. In certain embodiments, an FB chip 620 is coupled to MCM package 190, and one or more memory chips comprising the memory subsystem are coupled to the FB chip 620 to form a chip stack.

MCM 600 may also include a system functions chip 618, configured to implement functionality required by MCM 600, but not implemented in GPC chips 610, FB chips 620, or Interconnect 614. For example, system functions chip 618 may implement power management functions, interface functions, system control functions, and watchdog functions, or any combination thereof in conjunction with the operation of GPC chips 610. System functions chip 618 may be coupled to one or more GPC chips 610, one or more FB chips 620, interconnect 614, or any technically feasible combination thereof. In one embodiment, system functions chip 618 implements a host interface. Signals associated with the host interface may be routed to a host system through MCM package 190 to external pins, such as external solder balls coupled to MCM package 190. In certain embodiments, system functions chip 618 also implements one or more display controllers, configured to provide raster refresh to a display device such as an LCD panel. Video signals may be routed to corresponding display devices through MCM package 190 to external pins.

FIG. 6B illustrates a directly-connected multi-chip graphics processing unit implemented as MCM 600, in accordance with one embodiment. As shown, interconnect 614 is configured to directly connect each GPC chip 610 to each FB chip 620. In the directly-connected topology shown in FIG. 6B, each interconnect link 612 illustrated in FIG. 6A comprises direct-connection links organized as channels, with one channel connecting one GPC chip 610 to one FB chip 620. In one embodiment, a given link associated with interconnect link 612 comprises a pair of GRS transceivers. A first GRS transceiver of the pair of GRS transceivers is included in a GPC chip 610, and a second GRS transceiver of the pair of GRS transceivers is included in an FB chip 620. Associated electrical traces manufactured within MCM package 190 couple the first GRS transceiver to the second GRS transceiver to complete the link. In one embodiment, system functions chip 618 is coupled to one or more GPC chip 610 through a corresponding interconnect link 616. Interconnect link 616 may comprise single-ended signals, differential signals, or any technically feasible combination thereof. In one embodiment, interconnection link 616 comprises GRS signals. In such an embodiment, at least one GRS transceiver is manufactured within system functions chip 618 and coupled to interconnection link 616. A second GRS transceiver may be manufactured within a GPC chip 610, or any other device coupled to interconnection link 616.

In one embodiment, each FB chip 620 is coupled to a corresponding memory subsystem (not shown) through an associated memory link 624. In certain embodiments, each memory link 624 comprises a pair of GRS transceivers. A first GRS transceiver of the pair of GRS transceivers is included in an FB chip 620, and a second GRS transceiver of the pair of GRS transceivers is included in a chip comprising a corresponding memory subsystem. Associated electrical traces manufactured within MCM package 190 couple the first GRS transceiver to the second GRS transceiver to complete the link. As described previously, the memory subsystem may comprise at least one memory chip, such as a DRAM or cache memory chip. The at least one memory chip may be assembled into a stack.

In one embodiment, illustrated below in FIG. 6F, a given FB chip 620 is included within a vertical stack of chips that includes a corresponding memory subsystem. In certain embodiments, an FB chip 620 is coupled to MCM package 190, and one or more memory chips comprising the memory subsystem are coupled to the FB chip 620 to form a chip stack.

In one embodiment, each FB chip 620 is configured to transmit a memory access protocol over a corresponding memory link 624 that specifies, without limitation, a memory read request configured to include an access address, and a memory write request configured to include an access address and write data. In one embodiment, each memory link 624 implements the memory access protocol described in conjunction with FIG. 6A. In certain embodiments, interconnection links 612, 622, 616 may each comprise a primary communications interface between two devices. In one embodiment, a primary communications interface is defined as one interface, such as a set of one or more GRS transceivers, which is configured to provide the highest measure of bandwidth among one or more interfaces fabricated within the same device. In another embodiment, a primary communications interface is defined as an interface that is configured to convey memory access requests and associated data. In yet another embodiment, a primary communications interface is defined an interface that is configured to convey data, such as shader program data or application data, between two devices. In a preferred embodiment, each primary communications interface implements a single-ended signaling regime.

FIG. 6C illustrates a hub-connected multi-chip graphics processing unit implemented as MCM 600, in accordance with one embodiment. As shown, compared with FIG. 6A, the interconnect 614 is replaced with a hub chip 640 that is coupled to each GPC chip 610 and to each FB chip 620. In the hub-connected topology shown in FIG. 6C, each interconnect link 612 represents a connection between a GPC chip 610 and hub chip 640. Similarly, each interconnect link 622 represents a connection between an FB chip 620 and hub chip 640. In one embodiment, a given interconnect link 612 comprises a pair of GRS transceivers. A first GRS transceiver of the pair of GRS transceivers is included in a GPC chip 610, and a second, corresponding GRS transceiver is included in hub chip 640. Associated electrical traces manufactured within MCM package 190 couple the first GRS transceiver to the second GRS transceiver to complete the link. In one embodiment, system functions chip 618 is coupled to hub chip 640 through interconnect link 616.

In one embodiment, each FB chip 620 is coupled to a memory subsystem (not shown) through a corresponding memory link 624. In one embodiment, each memory link 624 comprises a pair of GRS transceivers. A first GRS transceiver of the pair of GRS transceivers is included in an FB chip 620, and a second GRS transceiver of the pair of GRS transceivers is included in a chip comprising a corresponding memory subsystem. Associated electrical traces manufactured within MCM package 190 couple the first GRS transceiver to the second GRS transceiver to complete the link. As described previously, memory subsystem may comprise at least one memory chip, such as a DRAM or cache memory chip. The at least one memory chip may be assembled into a stack.

Hub chip 640 may implement any technically feasible internal communication topology, such as a crossbar, ring, butterfly, flattened butterfly, Clos, or general mesh network to interconnect links 612, 622. Any technically feasible admission control and arbitration mechanism may be implemented for managing and arbitrating ingress to egress traffic. Although MCM 600 is shown in FIG. 6C as comprising four GPC chips 610(0)-610(3) and four FB chips 620(0)-620(3), any number of GPC chips and any number of FB chips may be included within MCM 600 and coupled to hub chip 640. Similarly, any number of memory subsystems may be included within MCM 600.

FIG. 6D illustrates a network-connected multi-chip graphics processing unit implemented as an MCM 600, in accordance with one embodiment. As shown, interconnect 614 comprises two router chips 650 coupled to each other, to associated GPC chips 610, and to associated FB chips 620. In the network-connected topology shown in FIG. 6D, each interconnect link 612, 622 represents a connection to a corresponding router chip 650. In one embodiment, a given interconnect link 612 comprises a pair of GRS transceivers. A first GRS transceiver of the pair of GRS transceivers is included in a GPC chip 610, and a second, corresponding GRS transceiver is included in a corresponding router chip 650. Associated electrical traces manufactured within MCM package 190 couple the first GRS transceiver to the second GRS transceiver to complete the link. In one embodiment, system functions chip 618 is coupled to one or more router chip 650 through interconnect link 616.

In one embodiment, each FB chip 620 is coupled to a memory subsystem (not shown) through a memory link 624. In one embodiment, each memory link 624 comprises a pair of GRS transceivers. A first GRS transceiver of the pair of GRS transceivers is included in a FB chip 620, and a second GRS transceiver of the pair of GRS transceivers is included in a chip comprising a corresponding memory subsystem. Associated electrical traces manufactured within MCM package 190 couple the first GRS transceiver to the second GRS transceiver to complete the link. As described previously, memory subsystem may comprise at least one memory chip, such as a DRAM or cache memory chip. The at least one memory chip may be assembled into a stack.

During normal operation, router chip 650(0) may receive a data packet from a source device for delivery to a destination device. The source device and the destination device may each separately comprise a GPC chip 610, an FB chip 620, a system functions chip 618, or any other technically feasible destination device. The data packet may comprise a read request, a write request, acknowledgement to a previous request, a data message, a command, or any other technically feasible unit of information. Router chip 650(0) is configured to forward the data packet to the destination device along a forwarding path. The forwarding path may include, without limitation, an interconnect link 612, an interconnect link 622, an inter-router link 652, or any technically feasible combination thereof. If the source device and the destination device are both directly coupled to router chip 650(0), then router chip 650(0) may forward the data packet directly from the source device to the destination device. If the destination device is instead directly coupled to router chip 650(1), then the router chip 650(0) forwards the data packet through inter-router link 652 to router chip 650(1), which then forwards the data packet to the destination device. In one embodiment, inter-router link 652 comprises a pair of GRS transceivers. A first GRS transceiver of the pair of GRS transceivers is included in a router chip 650(0), and a second GRS transceiver of the pair of GRS transceivers is included router chip 650(1). Associated electrical traces manufactured within MCM package 190 couple the first GRS transceiver to the second GRS transceiver to complete the link.

As shown, MCM 600 includes two router chips 650(0), 650(1), configured to form a two node multi-hop network. However, MCM 600 may include an arbitrary number of router chips 650, interconnected through a corresponding set of GRS transceivers to form an arbitrary multi-hop network topology such as a mesh, torus, butterfly, flattened butterfly, or Clos without departing the scope and spirit of embodiments of the preset invention.

A GRS transceiver within the source device includes a GRS transmitter that is configured to transmit serialized data comprising the data packet until the data packet is transmitted in full to a GRS receiver within the destination device or within an interconnection chip preparing to forward the data packet. The GRS transmitter may implement two or more degrees of multiplexing by implementing a corresponding number of GRS data drivers and appropriate clocking circuitry. The GRS transmitter may be configured to perform method 560 to generate individual bits comprising the serialized data for transmission. Exemplary GRS transmitters illustrated in FIGS. 1A-5B implement two to one multiplexing; however, persons skilled in the art will understand that arbitrary degrees of multiplexing may be similarly implemented without departing the scope and spirit of embodiments of the present invention. The destination device may deserialize the serialized bits to construct the access request. If the destination device is configured to serve as a bridge or hub, then the access request may be forwarded to a destination device for processing. In certain embodiments, method 560 of FIG. 5C is performed to generate GRS signals transmitted over one or more interconnect links 612, 622, one or more inter-router link 652, or any combination thereof.

In one embodiment, interconnect links 612, 622 and inter-router links 652 comprise electrical traces within MCM package 190. In certain embodiments, memory links 624 also comprise electrical traces within MCM package 190. Each electrical trace may comprise a conductive element affixed to a dielectric substrate, such as an organic substrate layer of MCM package 190. Each electrical trace may be configured to exhibit a controlled electrical impedance. In one embodiment, MCM package 190 comprises an organic substrate, a silicon substrate, or a ceramic substrate.

In one embodiment, the data packet is generated by the source device for transmission to a destination device for processing. Certain data packets comprise a set of request fields, including, without limitation, an address field, which may uniquely identify the destination device and a specific address within the destination device. The access request is transmitted over a GRS interconnect to the destination device.

In one embodiment, a non-transitory computer readable medium is configured to represent a detailed design of MCM package 190, including all electrical connections. Such electrical connections include electrical traces designed to support ground-referenced single-ended signals, including, without limitation, interconnect links 612, 622, memory links 624, and inter-router link 652. Each GRS interconnect may include an abstract representation of connectivity, such as connectivity represented within a net list. Individual traces may be represented as code within a net list file. Persons skilled in the art will understand that many net list formats are available, and any technically feasible non-transitory computer readable medium configured to represent system-on-package 600 is within the scope and spirit of the present invention.

FIG. 6E illustrates a network-connected multi-chip graphics processing unit with frame buffer-attached memory implemented as an MCM 600, in accordance with one embodiment. As shown, a memory subsystem 630 is coupled directly to each FB chip 620 of FIG. 6D through a corresponding memory link 624. Each memory subsystem 630 may include, without limitation, one or more DRAM chips, one or more cache memory chips, a shim interface chip, or any technically feasible combination thereof.

In one embodiment, illustrated below in FIG. 6F, a given FB chip 620 is included within a vertical stack of chips that includes a corresponding memory subsystem. In certain embodiments, an FB chip 620 is coupled to MCM package 190, and one or more memory chips comprising the memory subsystem are coupled to the FB chip 620 to form a chip stack.

FIG. 6F depicts a cross-section of an MCM 600 configured to implement a chip stack 670 comprising an FB chip 620 and a memory subsystem 630, in accordance with one embodiment. As shown, chip stack 670 includes an FB chip 620, such as FB chip 620(0) of FIG. 6A, 6B, 6C, 6D, or 6E, and a memory subsystem 630, such as memory subsystem 630(0) of FIG. 6E. FB chip 620 is coupled to a client chip 672 through interconnect links 622, fabricated within MCM package 190. FB chip 620 is also coupled to one or more memory chips comprising memory subsystem 630 through memory link 624, within chip stack 370.

In one embodiment, client chip 672 comprises a GPC chip 610, as illustrated in FIG. 6B. In another embodiment, client chip 672 comprises hub chip 640 of FIG. 6C. In yet another embodiment, client chip 672 comprises a router chip 650 of FIG. 6D or 6E.

A set of solder balls 660 coupled to MCM package 190 are configured to couple a set of signals to a host system (not shown). The set of signals may include, without limitation, a host interface bus and one or more video buses. The solder balls may also couple power and ground networks associated with MCM 600 to corresponding power and ground networks provided by the host system.

In one embodiment of the directly-connected topology illustrated in FIG. 6B, each GPC chip 610 is manufactured to include at least one additional channel within interconnection link 612 and each FB chip 620 is manufactured to include at least one additional channel within interconnection link 622. In this embodiment, each additional channel is substantially functional, but remains unconnected to another device. For example, GPC chips 610(0)-610(3) of FIG. 6B may be manufactured to include five channels within interconnection link 612; however, one of the five channels is not coupled to a corresponding FB chip 620 through interconnect 614. In a second embodiment of the directly-connected topology, a fifth GPC chip 610 is advantageously added to MCM 600, along with a fifth FB chip 620. Here, the additional channel within interconnection link 612 is coupled through interconnect 614 to the additional channel within interconnection 622.

In certain embodiments, MCM 600 is designed to accommodate a maximum number of GPC chips 610 corresponding to a number of channels within interconnection link 612 and a corresponding number of FB chips 620, configured to include the same number of channels within interconnection link 622. In one configuration, MCM 600 includes a certain number of GPC chips 610 and corresponding FB chips 620 that is less than the maximum number of GPC chips 610. In a different configuration, MCM 600 includes the maximum number of GPC chips 610 and corresponding FB chips 620. In this way, overall GPU performance may be scaled by populating more or fewer GPC chips 610 and FB chips 620 within MCM 600. Because specifying which footprints within MCM 600 should be populated with components represents significantly less engineering effort than redesigning a typical GPU chip, this approach to scaling GPU performance results in significant development efficiencies over the prior art. In other embodiments, MCM 600 may be designed to accommodate a specific number of GPC chips 610 and FB chips 620, based on prevailing GPU requirements. In such embodiments, significant development efficiencies may be achieved because designing an MCM package represents significantly less engineering effort than designing a different configuration of GPU.

In another embodiment, hub chip 640 of FIG. 6C is manufactured to include a maximum number of interconnection links 612 and interconnection links 622. Each interconnection link 612 may be coupled to a GPC chip 610, while each interconnection link 622 may be coupled to an FB chip 620. In such an embodiment, the number of interconnection links 612, 622 available from hub chip 640 determines a maximum number of GPC chips 610 and FB chips 620. In one configuration of MCM 600, less than the maximum number of GPC chips 610 are included in MCM 600 along with corresponding FB chips 620. In a second configuration, a maximum number of GPC chips 610 and corresponding FB chips 620 are included in MCM 600. In such embodiments, significant development efficiencies may be achieved because configuring of designing an MCM package represents significantly less engineering effort than designing a different configuration of GPU.

In yet another embodiment, router chips 650 of FIG. 6D are manufactured to include a maximum number of interconnection links 612 and interconnection links 622. Each interconnection link 612 may be coupled to a GPC chip 610, while each interconnection link 622 may be coupled to an FB chip 620. In such an embodiment, the number of interconnection links 612, 622 collectively available from router chips 650 within MCM 600 determines a maximum number of GPC chips 610 and FB chips 620. In one configuration of MCM 600, less than the maximum number of GPC chips 610 are included in MCM 600 along with corresponding FB chips 620. In a second configuration, a maximum number of GPC chips 610 and corresponding FB chips 620 are included in MCM 600. In such embodiments, significant development efficiencies may be achieved because configuring of designing an MCM package represents significantly less engineering effort than designing a different configuration of GPU.

In one embodiment, MCM 600 is configured to include a general purpose central processing unit (CPU). The CPU may be coupled to interconnect 614, one or more GPC chips 610, one or more FB chips 620, system functions chip 618, or any technically feasible combination thereof.

FIG. 6G illustrates a multi-chip graphics processing unit configured to include a processing chip 675 and a system functions chip 618 within MCM 600, in accordance with one embodiment. As shown, MCM 600 is configured to include the processing chip 675, and system functions chip 618. The processing chip 675 includes GPC circuits 611, FB circuits 621, and interconnect 614. Each GPC circuit 611 is configured to implement processing functionality of GPC chip 610 of FIG. 6A. Each FB circuit 621 is configured to implement processing and interface functionality of FB chip 620. System functions chip 618 is coupled to processing chip 675. GPC circuits 611 may be coupled to interconnect 614 through interconnect links 612, and FB circuits 621 may be coupled to interconnect 614 through interconnect links 622. In one embodiment, electrical traces associated with interconnect links 612 and interconnect links 622 are manufactured within MCM 600. In one embodiment, system functions chip 618 is coupled to processing chip 675 through interconnect link 616. In certain embodiments, system functions chip 618 is fabricated using a different fabrication technology than processing chip 675.

In one embodiment, processing chip 675 also includes one or more CPU cores (not shown). The one or more CPU cores may be coupled to interconnect 614 and configured to execute programming instructions residing within any technically feasible memory system, such as one or more memory sub-systems or system memory associated with a host. In an alternative embodiment, MCM 600 includes one or more CPU chips (not shown) that are coupled to the processing chip 675.

FIG. 6H illustrates a multi-chip graphics processing unit configured to accommodate an additional processing chip within MCM 600, in accordance with one embodiment. As shown, MCM 600 is configured to include a processing chip 677, and system functions chip 618. MCM 600 may also include one or more additional processing chip, such as GPC chip 610(0) and FB chip 620(0). Processing chip 677 comprises functions and circuits associated with processing chip 675 of FIG. 6G, with the addition of one or more interconnect links 612, 622 that are configured to connect interconnect 614 to additional processing chip, such as GPC chip 610(0) or 620(0). In one embodiment, system functions chip 618 is coupled to processing chip 677 through interconnect link 616. In certain embodiments, system functions chip 618 is fabricated using a different fabrication technology than processing chip 677.

In one embodiment, MCM 600 includes processing chip 677 and system functions chip 618. In another embodiment, MCM 600 includes processing chip 677, system functions chip 618, and GPC chip 610(0). In yet another embodiment, MCM 600 includes processing chip 677, system functions chip 618, GPC chip 610(0), and FB chip 620(0). In still yet another embodiment, MCM 600 includes processing chip 677, system functions chip 618, and FB chip 620(0). In each embodiment, MCM 600 may also include memory chips comprising one or more memory sub-systems.

FIG. 7 illustrates an exemplary system 700 in which the various architecture and/or functionality of the various previous embodiments may be implemented. As shown, a system 700 is provided including at least one central processor 701 that is connected to a communication bus 702. The communication bus 702 may be implemented using any suitable protocol, such as PCI (Peripheral Component Interconnect), PCI-Express, AGP (Accelerated Graphics Port), HyperTransport, or any other bus or point-to-point communication protocol(s). The system 700 also includes a main memory 704. Control logic (software) and data are stored in the main memory 704, which may take the form of random access memory (RAM). In one embodiment, graphics processor 706, and at least a portion of main memory 704 comprise MCM 600 of FIGS. 6A-6F.

The system 700 also includes input devices 712, a graphics processor 706, and a display 708, i.e. a conventional CRT (cathode ray tube), LCD (liquid crystal display), LED (light emitting diode), plasma display or the like. User input may be received from the input devices 712, e.g., keyboard, mouse, touchpad, microphone, and the like. In one embodiment, the graphics processor 706 may include a plurality of shader modules, a rasterization module, etc. Each of the foregoing modules may even be situated on a single semiconductor platform to form a graphics processing unit (GPU).

In the present description, a single semiconductor platform may refer to a sole unitary semiconductor-based integrated circuit or chip. It should be noted that the term single semiconductor platform may also refer to multi-chip modules with increased connectivity which simulate on-chip operation, and make substantial improvements over utilizing a conventional central processing unit (CPU) and bus implementation. Of course, the various modules may also be situated separately or in various combinations of semiconductor platforms per the desires of the user.

The system 700 may also include a secondary storage 710. The secondary storage 710 includes, for example, a hard disk drive and/or a removable storage drive, representing a floppy disk drive, a magnetic tape drive, a compact disk drive, digital versatile disk (DVD) drive, recording device, universal serial bus (USB) flash memory. The removable storage drive reads from and/or writes to a removable storage unit in a well-known manner. Computer programs, or computer control logic algorithms, may be stored in the main memory 704 and/or the secondary storage 710. Such computer programs, when executed, enable the system 700 to perform various functions. The main memory 704, the storage 710, and/or any other storage are possible examples of computer-readable media.

In one embodiment, the architecture and/or functionality of the various previous figures may be implemented in the context of the central processor 701, the graphics processor 706, an integrated circuit (not shown) that is capable of at least a portion of the capabilities of both the central processor 701 and the graphics processor 706, a chipset (i.e., a group of integrated circuits designed to work and sold as a unit for performing related functions, etc.), and/or any other integrated circuit for that matter.

Still yet, the architecture and/or functionality of the various previous figures may be implemented in the context of a general computer system, a circuit board system, a game console system dedicated for entertainment purposes, an application-specific system, and/or any other desired system. For example, the system 700 may take the form of a desktop computer, laptop computer, server, workstation, game consoles, embedded system, and/or any other type of logic. Still yet, the system 700 may take the form of various other devices including, but not limited to a personal digital assistant (PDA) device, a mobile phone device, a television, etc.

Further, while not shown, the system 700 may be coupled to a network (e.g., a telecommunications network, local area network (LAN), wireless network, wide area network (WAN) such as the Internet, peer-to-peer network, cable network, or the like) for communication purposes.

In one embodiment, certain signals within bus 702 are implemented as GRS signals, as described above in FIGS. 1A-6F.

While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 

What is claimed is:
 1. A system, comprising: a first processor chip configured to include a first single-ended signaling interface circuit; a graphics processing cluster (GPC) chip that includes a multi-threaded processor core configured to execute graphics shader programs and a second single-ended interface circuit; a multi-chip module (MCM) package configured to include the first processor chip, the GPC chip, and an interconnect circuit, wherein the interconnect circuit comprises a first interconnect chip and a second interconnect chip that is coupled to the first interconnect chip and configured to transmit an access request from a second processor chip to the first interconnect chip; a first set of electrical traces fabricated within the MCM package and configured to couple the first single-ended signaling interface circuit to the interconnect circuit; a second set of electrical traces fabricated within the MCM package and configured to couple the second single-ended signaling interface circuit to the interconnect circuit; a memory controller chip that is configured to perform read-modify-write operations to a frame buffer memory and includes a third single-ended signaling interface circuit, wherein the first interconnect chip is configured to transmit the access request to the memory controller chip and the memory controller chip is configured to reply to the access request; and a third set of electrical traces fabricated within the MCM package and configured to couple the third single-ended signaling interface circuit to the interconnect circuit.
 2. The system of claim 1, wherein the first single-ended signaling interface circuit comprises a first ground-referenced single-ended signaling (GRS) interface circuit and the second single-ended signaling interface circuit comprises a second GRS interface circuit.
 3. The system of claim 2, wherein the first GRS interface circuit comprises: a first GRS driver circuit, configured to: pre-charge a first capacitor to store a first charge during a first pre-charge phase; and drive an output signal relative to a ground network based on the first charge during a first drive phase; a second GRS driver circuit, configured to: pre-charge a second capacitor to store a second charge during a second pre-charge phase; and drive the output signal relative to a ground network based on the second charge during a second drive phase; and a receiver circuit, configured translate a ground-referenced single-ended input signal to a corresponding logic signal, wherein the first set of electrical traces comprise the input signal, the output signal, and the ground network.
 4. The system of claim 1, wherein the first processor chip comprises a graphics processing unit.
 5. The system of claim 1, wherein the memory controller chip comprises a memory interface circuit and a raster operations unit configured to execute graphics blend operations.
 6. The system of claim 1, further comprising a memory subsystem that is included within the MCM package and coupled to the memory controller chip.
 7. The system of claim 6, wherein the memory subsystem comprises at least two stacked chips.
 8. The system of claim 6, wherein the memory subsystem and the memory controller chip are configured to form a vertical stack.
 9. The system of claim 1, wherein the first processor chip is manufactured from a first fabrication process and the memory controller chip is manufactured from a second fabrication process.
 10. The system of claim 1, further comprising: an interface shim chip that includes a memory controller circuit; and at least one memory chip coupled to the memory controller circuit, wherein the memory controller circuit transmits data associated with memory access requests between the shim chip and the at least one memory chip.
 11. The system of claim 1, wherein the second processor chip is configured to include a fourth single-ended signaling interface circuit, and further comprising a fourth set of electrical traces fabricated within the MCM package and configured to couple the fourth single-ended signaling interface circuit to the interconnect circuit.
 12. The system of claim 1, wherein at least one interface channel associated with the first interconnect chip is configured to be unconnected when coupled to the MCM package.
 13. The system of claim 1, wherein the interconnect circuit comprises the first set of electrical traces and the second set of electrical traces.
 14. The system of claim 1, further comprising a system functions chip configured to generate a video refresh associated with data stored within one or more memory subsystems included within the MCM package.
 15. The system of claim 1, wherein the MCM package comprises an organic substrate.
 16. The system of claim 1, further comprising a central processing unit that is included within the MCM package.
 17. A system, comprising: a first processor chip configured to include a first single-ended signaling interface circuit; a graphics processing cluster (GPC) chip that includes a multi-threaded processor core configured to execute graphics shader programs and a second single-ended interface circuit; a multi-chip module (MCM) package configured to include the first processor chip, the GPC chip, and an interconnect circuit, wherein at least one interface channel associated with the first processor chip is configured to be unconnected when coupled to the MCM package; a first set of electrical traces fabricated within the MCM package and configured to couple the first single-ended signaling interface circuit to the interconnect circuit; and a second set of electrical traces fabricated within the MCM package and configured to couple the second single-ended signaling interface circuit to the interconnect circuit.
 18. A non-transitory computer readable medium, comprising: code representing a first set of electrical traces configured to couple a first single-ended signaling interface circuit of a first processor chip to a interconnect circuit within a multi-chip module (MCM) package, wherein at least one interface channel associated with the first processor chip is configured to be unconnected when coupled to the MCM package; and code representing a second set of electrical traces configured to couple a second single-ended signaling interface circuit of a graphics processing cluster (GPC) chip that includes a multi-threaded processor core configured to execute graphics shader programs to the interconnect circuit within the MCM package, wherein the first set of electrical traces and the second set of electrical traces comprise ground-referenced single-ended (GRS) signal lines.
 19. The non-transitory computer readable medium of claim 18, wherein the first single-ended signaling interface circuit comprises a first ground-referenced single-ended signaling (GRS) interface circuit and the second single-ended signaling interface circuit comprises a second GRS interface circuit.
 20. The non-transitory computer readable medium of claim 18, wherein the interconnect circuit comprises the first set of electrical traces and the second set of electrical traces. 